Decoking of pebbles in pebble heater cracking of hydrocarbons



Patented Dec. 22, 1953 DECOK G 0F 'PEBBLES IN PEBBLE HEATER CRACK NG OFHYDROCARBONS Myron 0. Kilpatrick, Bartlesville, Okla, assignor toPhillips ,Petroleum Company, a. corporation of Delaware Application July10, 1950, Serial No. 172,986

13 Claims.

This invention relates to hydrocarbon conversion involving cracking andcoke deposition in pebble heater type apparatus and to methods ofremoving coke from the system, particularly from the pebbles.

Pebble heater apparatus is finding increasing favor in the conversion ofhydrocarbons at elevated temperatures in the range of 1500 to about 2500F. because of the fast heating rate-continuous operation, and otherprocess advantages prochamber with a countercurrent stream of thehydrocarbon to be reacted, or converted, so as to heat the same toreaction temperature and supply the heat of reaction required to effectthe conversion. The pebbles then gravitate from the lower end of thereactor to the bottom of an ele- 5 vator which lifts them to a pointabove the top of the pebble heater chamber from whence they gravitate tothat chamber for reheating and gravitation to the reaction chamber. Thesystem from the pebble inlet in the heating chamber to a pebble feederdevice in the conduit between the bottom of the reaction chamber and thelower end of the elevator is maintained full of a compact gravitatingmass of pebbles at all times durins o e ion of the u t- When efllectinghydrocarbon conversion involving cracking in a pebble heater, it hasbeen found that the pebbles and certain areas in the reactor acquirecoke deposits which eventually in terfere with pebble circulation andcause shutcoking of pebbles in the conversion of hydrocarbons in apebble heater apparatus that this invention is primarily concerned. Oneof the principal objects of the invention is to remove coke deposited onthe pebbles during hydrocarbon conversion before pebble flow isappreciably impeded or sto ped. Another object of the invention is toremove coke deposited in the reactor of a pebble heater. It is also anobject of the invention to increase the onstream time in the pebbleheater cracking of hydrocarbons. Other objects of the invention willbecome apparent from a consideration of the accompanying disclosure.

The invention provides for the removal of coke from the pebbles in thepebble heater conversion of hydrocarbons by combustion of the coke inthe pebble heating chamber while diminishing the coke deposition in thereactor by decreasing the how of hydrocarbon therethrough andsupplementing the hydrocarbon feed with a non-deleterious, non-cokinggas such as steam, regeneration gas, flue gas, CO2, nitrogen, ormethane. In normal pebble heater conversion of hydrocarbons whichinvolves deposition of coke on the pebbles, a portion of the coke isburned 01f due to the presence of an excess of oxygen in the combustiongas passing through the pebble heating chamber. However, aftercontinuous operation for a period of a week or more it is often timesfound that the rate of formation of coke in the reactorand on thepebbles is such that an accumulation of carbon takes place to such anextent that coke and pebble clusters or agglomerates form in the reactorand interfere with the free flow or gravitation of pebbles from thebottom of the reactor to the bottom of the elevator and particularlythrough the pebble feeder.

The invention provides for a catching-up period on the removal of cokefrom the pebbles in the system and from the reactor. In accordance withthe invention, the flow of hydrocarbon through the reactor is diminishedappreciably or out off entirely and supplemented or replaced with one ofthe non-coking gases enumerated hereinbefore, and the. coke burningprocess in the pebble heating chamber is continued until the pebbles arerelatively free of. coke deposit. The non-coking gas passing through thereactor during this operation functions to control the temperature ofthe pebbles so that they are not too hot for the ele vator apparatus:and ducts between the reactor and the. pebble preheater. When utilizingsteam and when the temperature is sufficiently high in the reactor-,thisnon-coking gas serves a dual purpose including the removing of cokedeposit from the reactor dome and other surfaces in the reactor by thewell known water gas reactionand possibly by some vaporization of heavytarry polymers. When the temperature of the pebbles coming into thereactor is upwards of about 2100 F. the temperature in the upper sectionof the reactor can be maintained above about 2000 F. which will supportthe water gas reaction.

In cases where the coke deposit is suiliciently heavy that the burningthereof in the pebble heat ing chamber in conjunction with the heatingef fected by the combustion of fuel and oxygen therein raises thetemperature of the pebbles considerably above the operating temperaturethereof during the onstream conversion of hydrocarbon, the amount ofcombustion can be reduced by decreasing the fuel supply to thecombustion chamber in the lower portion of the pebble heating chamber soas to effectively reduce the amount of heat imparted to the pebbles inthis chamber.

The flow of gas through the reactor during the coke removal phase of theoperation is preferably regulated so that the temperature of theoutgoing pebbles is about the same as the temperature dur inghydrocarbon conversion, i. e., in the range of about 500 to 950 F. It isalso desirable during this period to regulate the heating in the pebbleheating chamber and the flow of gas through the reactor so that thepebbles coming into the reactor at the end of the coke removal phase issuitable for the hydrocarbon conversion being effected.

It is not necessary in most instances to entirely out off the supply ofhydrocarbon feed to the reactor during the coke removal step, but itusually essential to reduce the hydrocarbon feed to below 60 weight percent of the feed on normal onstream operation. For the most rapidremoval. of the coke from the pebbles by combustion in the pebbleheating chamber it is essential to completely supplement the hydrocarbonfeed with non-deleterious, non-coking gas.

In some types of operation it has been found expedient to flowconsiderable superheated steam into the dome of the reactor during thehydrocarbon conversion phase of operation. Even when using thistechnioue it is found necessary to remove coke from the pebbles in somemanner and the process of the invention has been found most efiective.

Forthe purpose of the invention, methane is regarded as a non-coking gassince it can readily be passed through the reactor at a rate whichavoids appreciable coke deposition. Combustion gas for the feed to thereactor during the coke removal step is readily obtained from the pebbleheating chamber eiiluent. Nitrogen or CO2 from this source can also beused to advantage when desired.

For a more complete comprehension of the invention, reference may be hadto the drawing which shows diagrammatically an arrangement of pebbleheater apparatus and flow therethrough in accordance with the invention.

Referring to the drawing, pebbles in pebble heating chamber H are heatedby contact with hot combustion gas formed in the bottom of the chamberby means of burner 12 to which is fed a combustible mixture of air andfuel through lines lt and E l, respectively. The hot combustion gaspasses upwardly through the gravitating bed of pebbles in chamber H andpasses out stack l6. Line H serves to introduce supplemental air orother oxygen-containing gas during the coke burning phase of operationor this additional supply of oxygen can be supplied through line 13, ifdesired. By proper operation of the valves shown in lines i3, Hi, andll, the proportions of oxygen and fuel can be regulated in any desirablemanner.

After the pebbles are heated in chamber ll,

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they descend through an interconnecting threat is to reactor l9 wherethey are contacted with a suitable hydrocarbon feedintroduced through adistributing member (not shown) in the bottom of the chamber throughline 2%. The hydrocarbon effiuent from reactor it is taken off throughline 22. Line 23 serves to introduce the non-deleterious, non-coking gasto be supplied to the reactor either alone or in admixture withhydrocarbon fed through line 2E. The efiiuent non-coking gas may betaken off through line 25 by proper manipulation of the valves in lines22 and 2 when it is desired to avoid passing the non-coking gas into thesame separation or treating system as is used for the eifiuenthydrocarbon.

The hot pebbles entering reactor it pass downwardly therethrough and arecooled by transmission of sensible heat to the incoming hydrocarbon ieedand/or non cosing so that the temperature is below about 958 to permittheir transfer through the duct and elevator system to the top ofpreheater ii. In normal operation, pebble feeder 26 in duct 2? is op in@011- junction with elevator 23 to maintain a level of pebbles in thetop of pebble heating chamber H, thereby maintaining this chambertogether with throat i8 and reactor is filled with compact stream ofpebbles. Pebble duct serves to deliver pebbles from the top of theelevator to the pebble inlet to preheater H Elevator 28 may be of thescrew, buck t, or gas liit type.

Example In a commercial pebble heater process in which average ofapproximately 79,060 s. c. f. h. of a fractionator overhead havingaverage composition of 20 methane, 58% ethane, and 22% propane (byweight) was fed into the bottom of the reactor, the cracking temperaturewas main-' tained at an average of about by feeding pebbles into thereactor at an arc temper ature of about 1906" F. at an average flow rateof about lLQiiO pounds per hour.

During the seventh day of continuous opera tion, the ammeter on theelevator motor indicated an irregular and subnorinal pebble flow to theelevator and it was found that clust s of pebbles and coke wereinterfering with 'ular pebble flow. The flow of hydrocarbon v .sgradually replaced with steam, the fuel supply to the pebble heaterburner was reduced after a short period so as to control thetemperature, the air supply to the burner was increased so as to burnoif coke from the pebbles in the pebble heating chamber. After about 8hou s of this coke burning operation the pebbles were found to besubstantially free of coke and onstream cracking of hydrocarbons wasresumed. During the coke burning phase of the cycle and particularly inthe latter stages, the temperature of the pebbles passing into thereactor was adjusted to that required in the onstream conversion ofhydrocarbon.

During the process, maximum ethylene and propylene production amountedto 53,000 and 6,000 pounds per day, respectively.

The illustrative details set forth herein are not to be construed asimposing unnecessary limitations upon the invention, the scope of whichis set forth in the claims.

I claim:

1. In a process for conversion of hydrocarbons in a pebble heater inwhich refractory heat carrying pebbles are successively heated to atemperature above conversion temperature in the range of 1500 to 2500 F.in an upper chamber by contact with hot combustion gas, gravitated to alower chamber in which they are contacted with a stream of hydrocarbonunder cracking conditions so as to deposit coke on said pebbles andgradually cause pebble and coke agglomerates to form, and then saidpebbles are transferred to said upper chamber for reheating; the methodof removing said cokefrom said pebbles which comprises discontinuing theflow of hydrocarbon through said lower chamber and passing therethrougha stream of non-deleterious, non-coking gas under the conditions in thereaction zone while simultaneously contacting the pebbles in said upperchamber with an oxygen-containing gas in excess of that present in saidupper chamber prior to discontinuing the flow of hydrocarbon to thelower chamber so as to burn coke therefrom; continuing the flow of saidlast-named gases and said pebbles at least until the major portion ofthe coke on said pebbles has been removed; and thereafter discontinuingthe flow of said gases and resuming said hydrocarbon conversion andheating of said pebbles.

2. The process of claim 1 in which said nondeleterious, non-coking gasis steam.

3. The process of claim 1 in which said nondeleterious, non-coking gasis methane.

4. The process of claim 1 in which said nondeleterious, non-coking gasis combustion gas.

5. The process of claim 1 in which said nondeleterious, non-coking gasis nitrogen.

6. The process of claim 1 in which said nondeleterious, non-coking gasis 002.

7. In a process for conversion of hydrocarbons in a pebble heater inwhich refractory heat carrying pebbles are successively heated to atemperature above a conversion temperature in the range of 1500 to 2500F. in an upper chamber by contact with hot combustion gas, gravitated toa lower chamber in which they are contacted with a stream of hydrocarbonunder cracking conditions so as to deposit coke on said pebbles andgradually cause pebble and coke agglomerates to form, and then saidpebbles are transferred to said upper chamber for reheating, the methodof removing said coke from said pebbles which comprises reducing theflow of hydrocarbon to less than 60 per cent of the onstream flow andsubstituting therefor at least an equal amount of a non-deleterious,non-coking gas under the conditions in the reaction zone whilecontinuing pebble flow through the system and contacting of pebbles insaid upper chamber with an oxygen-containing ga in excess of thatpresent in said upper chamber prior to reducing the flow of hydrocarbonto the lower chamber so as to burn oil? the major portion of the coke onsaid pebbles; and thereafter resuming the full onstream flow ofhydrocarbon in the lower chamber and combustion gas in the upperchamber.

8. The process of claim 7 in which the noncoking gas is steam.

9. The process of claim 7 in which the hydrocarbon is completelyreplaced with steam.

10. The process of claim 9 in which the temperature of the pebblesentering said lower chamber is maintained above 2000 F. so as to eifectthe water gas reaction between the coke and steam, thereby aiding in theremoval of coke from the system.

11. The process of claim 9 inwhich the temperature of the pebblesentering said lower chamber is maintained below 2000 F. andsubstantially all of the coke removed from said pebbles is by oxidationin the upper chamber.

12. In a process for conversion of hydrocarbons in a pebble heatersystem in which a gravitating stream of refractory heat carrying pebblesare successively heated to a temperature above conversion temperature inthe range of 1500 to 2500" F. by contact with combustion gas produced byburning a combustible mixture of fuel and excess oxygen in a combustionzone in the bottom of an upper pebble heating chamber, gravitated to alower chamber in which they are contacted with a stream of hydrocarbonunder cracking conditions so as to deposit coke on said pebbles andgradually cause pebble and coke agglomerates to to form, and saidpebbles are transferred from said lower to said upper chamberfor'reheating, the method of removing said coke from said pebbles whichcomprises diminishing the flow of said hydrocarbon and supplementing thesame with a non-deleterious, non-coking gas under the conditions in thereaction zone While continuing the flow of pebbles through the systemand decreasing the proportion of fuel in said combustible mixture so asto provide sufiicient excess of oxygen to burn off coke from saidpebbles; continuing the preceding operation until the major portion ofcoke is burned from said pebbles in the entire system; and thereafterresuming the full onstream flow of hydrocarbon and combustion as.

13. The process of claim 12 in which the heating in the pebble heatingchamber and. cooling in the reactor are coordinated to effect a pebbleentrance temperature to the reactor at the level of their conversioninlet temperature.

MYRON O. KILPATRICK.

References Cited in the file of this patent UNITED STATES PATENTS NumberName Date 2,862,621 Fahnestock Nov. 14, 1944 2,464,810 Hirsch et al Mar.22, 1949 2,543,742 Evans Feb. 27, 1951

1. IN A PROCESS FOR CONVERSION OF HYDROCARBONS IN A PEBBLE HEATER INWHICH REFRACTORY HEAT CARRYING PEBBLES ARE SUCCESSIVLEY HEATED TO ATEMPERATURE ABOVE CONVERSION TEMPERATURE IN THE RANGE OF 1500 TO 2500*F. IN AN UPPER CHAMBER BY CONTACT WITH HOT COMBUSTION GAS, GRAVITATED TOA LOWER CHAMBER IN WHICH THEY ARE CONTACTED WITH A STREAM OF HYDROCARBONUNDER CRACKING CONDITIONS SO AS TO DEPOSIT COKE ON SAID PEBBLES ANDGRADUALLY CAUSE PEBBLE AND COKE AGGLOMERATES TO FORM, AND THEN SAIDPEBBLES ARE TRANSFERRED TO SAID UPPER CHAMBER FOR REHEATING; THE METHODOF REMOVING SAID COKE FROM SAID PEBBLES WHICH COMPRISES DISCONTINUINGTHE FLOW OF HYDROCARBON THROUGH SAID LOWER CHAMBER; AND PASSINGTHERETHROUGH A STREAM OF NON-DELETERIOUS, NON-COKING GAS UNDER THECONDITIONS IN THE REACTION ZONE WHILE SIMULTANEOUSLY CONTACTING THEPEBBLES IN SAID UPPER CHAMBER WITH AN OXYGEN-CONTAINING GAS IN EXCESS OFTHAT PRESENT IN SAID UPPER CHAMBER PRIOR TO DISCONTINUING THE FLOW OFHYDROCARBON TO THE LOWER CHAMBER SO AS TO BURN COKE THEREFROM;CONTINUING THE FLOW OF SAID LAST-NAMED GAS AND SAID PEBBLES AT LEASTUNTIL THE MAJOR PORTION OF THE COKE ON SAID PEBBLES HAS BEEN REMOVED;AND THEREAFTER DISCONTINUING THE FLOW OF SAID GASES AND RESUMING SAIDHYDROCARBON CONVERSION AND HEATING OF SAID PEBBLES.